Microfabricated biological sensors based on acoustic devices combine a biologically active interface, which binds biological species (i.e., analytes) from an environment, with a physical transducer that provides an electrical output proportional to the amount of bound analyte. A commonly used acoustic device for biological sensing includes leaky surface acoustic wave (LSAW) sensors that rely on the electrical excitation of a shear-horizontal surface acoustic wave on a piezoelectric substrate. Typically, a wave is established on a surface and the collection of analyte mass on the surface influences the propagation of the surface wave. In particular, these analyte-induced changes can be sensed as variations in the velocity and amplitude of the surface wave.
Recently, Love wave sensors have received considerable attention for their high mass and viscous sensitivity with a minimal need for additional reagents. Minimizing the use of reagents is desirable for field deployable chem- and bio-detection systems. The transduction mechanism for Love wave sensors is based on propagating waves with a shear-horizontal (SH) polarization along the propagation direction. The SH polarization minimizes attenuation of the surface acoustic wave (SAW) into viscous media permitting detection in liquids. See G. Kovacs et al., Ultrason. Symp., pp. 281-285 (1992); G. Harding et al., Sensors Actuators A 61, 279 (1997); O. Tamarin et al., Biosensors and Bioelectronics 18, 755 (2003); and D. W. Branch and S. M. Brozik, Biosensors and Bioelectronics 19, 849 (2003).
Love wave sensors comprise a piezoelectric substrate that primarily excites SH waves which are subsequently confined by a thin guiding layer. In general, if the layer material loads the substrate (i.e., the shear velocity in the layer is smaller than in the substrate), the SH bulk mode will become a surface mode having a single, transverse component of displacement confined within a few wavelengths of the surface. In particular, at high frequencies, such that the wavelength is less than the layer thickness, a surface Love wave can be concentrated in the thin waveguide layer. Therefore, the waveguide layer is crucial to achieve high sensitivity by having a low shear velocity compared to the substrate. See G. Kovacs et al., Ultrason. Symp., 281 (1992); and Z. Wang and J. D. N. Cheeke, Appl. Phys. Lett. 64, 2940 (1994). For biodetection, the waveguide layer can also provide a mechanism for stable chemical attachment through covalent linkage of antibodies, DNA, or other biomolecules to achieve the required selectivity. Waveguide materials such as polymers, silicon dioxide (SiO2), and more recently zinc oxide (ZnO) are in use. See E. Gizeli et al., IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 39, 657 (1992); F. Herrmann et al., IEEE Trans. Ultrason. Ferroelect. Freq. Contr. 48, 268 (2001); and D. A. Powell et al., IEEE Ultrason. Symp. Proc., 493 (2002).
However, piezoelectric substrates that support such leaky surface acoustic waves, such as Love waves, require advanced transducer designs to avoid excitation of undesired modes. Unlike Rayleigh wave devices, where a true surface wave exists in the absence of dispersion, LSAW transducers require that bulk waves are suppressed and that intra-device acoustic reflections are minimized. Existing bidirectional transducers have major drawbacks in this regard since waves are launched in both the forward and backward directions and are complicated by bulk wave generation. Above about 100 MHz, the phase is highly non-linear and other modes interfere with the main SH sensing mode. Although edge reflections from backward traveling waves can be easily suppressed on substrates that support Rayleigh waves through the use of absorbers, this it not possible on substrates that support leaky waves. Moreover, since surface-skimming bulk waves (SSBW) propagate with a velocity very close to the leaky or shear horizontal mode on piezoelectric substrates, such as 36° YX lithium tantalate (LTO), the design of the transducer is highly critical to exciting the proper mode, especially at high frequencies. The design is further complicated by the fact that the electrode metal thickness determines the degree of propagation loss for leaky waves on LTO.
Therefore, a need exists for a SH surface acoustic wave (Love wave) sensor comprising a high-frequency interdigital transducer that provides low insertion loss and high out-of-band rejection, while suppressing bulk wave excitation at the stop band, to enable high sensitivity detection of biological and chemical analytes in a fluid.